The invention concerns new compounds and methods for the synthesis and purification of oligonucleotides, and more specifically, compounds and methods for synthesizing, chemically capping and purifying nucleic acids. Nucleic acids are of major importance in the living world as carriers and transmitters of genetic information. Since their discovery by F. Miescher they have aroused a wide scientific interest which has led to the elucidation of their function, structure and mechanism of action. Variations in nucleic acid sequence often account for differences in susceptibility to diseases and pharmacological responses to treatment. To illustrate, changes in a single base of a nucleic acid molecule, which are commonly referred to as single nucleotide polymorphisms (SNPs), can affect an individual's risk for a given disease. By comparing these variations, researchers are gaining an understanding of the medical utility of SNPs, thereby enhancing our ability to effectively diagnose, prognosticate, and treat disease. In addition, purified synthetic nucleotides are used for amplification in the polymerase chain reaction (PCR) and other amplification methods; as primers; hybridization probes for detection and/or sequencing, gene therapy, cloning, site-specific mutagenesis studies and the like. The quality of the result of these techniques is directly related to the purity of the oligonucleotides used.
As such, the purity of a nucleic acid molecule is crucial to elucidating the function and facilitating the manipulation of these molecules. Automated, solid phase synthesis is the most common approach for the production of short oligonucleotides. These synthetic methods are usually based on the stepwise reactions of phosphoramidite or H-phosphonate derivatives of nucleosides to form a continuous linkage of these monomeric building blocks in a pre-determined order (see e.g. T. Brown & D. J. S. Brown in Oligonucleotides and Analogues—A Practical Approach, (1991) (Eckstein, F., publ. IRL Press at Oxford University Press, Oxford, N.Y., Tokyo); Sambrook et al., 1989, Molecular Cloning—A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.; Oligonucleotide Synthesis (M. J. Gait, ed., 1984); Nucleic Acid Hybridization (B. D. Hames and S. J. Higgins. eds., 1984); Current Protocols in Nucleic Acid Chemistry, Beaucage, S. L.; Bergstrom, D. E.; Glick, G. D.; Jones, R. A., Eds., John Wiley & Sons, Inc.: New York, Chapters 1-4, 2000-2004; and a series, Methods in Enzymology (Academic Press, Inc.). The resulting oligonucleotides, however, are heterogeneous mixtures of sequences, which complicates purification and limits the scale on which oligonucleotides can be made and the resulting yield. The problem of purification is further increased as the length of the strand increases. Typically the resulting unreacted 5′-hydroxyl groups are chemically capped with acetic anhydride to prevent further chain elongation with an incorrect “failure” sequence. Another method, which can be performed in parallel, is the so-called trityl-on purification (TOP) which utilizes the lipophilicity of the trityl protecting group. The desired sequence carrying the lipophilic trityl group is retained on a lipophilic support material while failure sequences lacking the trityl group are removed. Following cleavage of the trityl group under acidic conditions, the product of the desired sequence can be eluted from the lipophilic support.
A variety of methods are used to purify oligonucleotides—the above mentioned reversed-phase chromatography, anion-exchange (AX) chromatography, polyacrylamide gel electrophoresis (PAGE), ethanol precipitation, or a combination of these techniques. However, these methods have the disadvantage in that both the acyl and trityl groups are relatively labile to the conditions employed in oligonucletide synthesis (e.g. typical oligonucleotide deprotection conditions involve incubation in aqueous ammonia at 55-60° C. for 16 hours) resulting in poor purification or low yields. These methods are also limited in that the hydrophobic interactions are not particularly strong, so the isolation efficiency decreases rapidly with increasing chain length. Consequently, these methods are limited to producing nucleotides of less than 100 nucleotides with low yields of the desired sequence.
Fluorous affinity strategies have been used for the purification of peptides (see Filippov et al Tetrahedron Lett. 2002, 43: 7809-7812; de Visser et al; Tetrahedron Lett 2003 44: 9013-9016; Montanari et al. J. Am. Chem. Soc. 2004, 126: 9528; Brittain et al. Nature Biotechnol. 2005 23: 463-468; Markowicz et al. Synthesis 2004 80-86; Mizuno et al. Chem. Lett 2005 34: 426-427), oligosaccharides (see Palmacci et al. Angew. Chem. Int. Ed 2001, 40: 4433; Manzoni Chem. Commun. 2003, 2930-2931 and Goto et al Synlett 2004, 2221-2223). Fluorous affinity strategies have also been used for the purification of oligonucleotides (see Pearson et al. J. Org. Chem. 2005 70: 7114-7122; Beller Helv. Chim. Acta 2005, 88: 171-179; Berry et al. WO 2006/081035, U.S. Pat. Publication No. 2006/0178507) although these reports disclose only use of fluorous trityl groups. As mentioned above, acetate and trityl capping groups often do not survive the deprotection conditions typically employed in oligonucleotide synthesis. In addition, Berry et al. use fluorous-DMTr to tag full length material. Their fluorous-purified materials are a distribution of the full-length product plus the expected deletion oligonucleotides (i.e., n-1, n-2, etc.), since the final phosphoramidite coupling attached a fluorous-capped nucleotide to a preexisting distribution of the desired chain plus deletion materials, which are not resolvable by HPLC, but can be detected by capillary electrophoresis analysis.
The present invention solves these problems by providing a phosphorous-based fluorous affinity cap to cap failure sequences, a method which can be used independent of the nucleoside used. The method uses a combination of fluorous capping and fluorous affinity chromatography that results in high yields and purities of non-capped oligonucleotide that are free of failure sequences even with long (>15mer) oligomers.